It is plausible that the models of our scientific theories correspond to possibilities. But exactly which models of which scientific theories stand in this correspondence? The answers to this question hinted at so far in the literature are too restrictive: they don't support the idea that the models of many of our best scientific theories correspond to physical possibilities. The paper thus provides a novel proposal for guiding belief about physical possibilities based on physics. The proposal draws on the notion (...) of an effective theory: a theory that applies very well to a particular, restricted domain. We argue that it is the models of effective theories that we should believe correspond, at least in part, to physical possibilities. It is thus effective theories that should guide modal reasoning in science. (shrink)